Unlike desktop-shell and kiosk-shell, the fullscreen-shell does not link
with the common shell-utils helpers. This is largely because
fullscreen-shell is largely in 'maintenance mode', seeing little more
than occasional bug fixes or changes required to accommodate new
interfaces.

This commit adds a dependency from fullscreen-shell to use the
shell-utils helper, in order to allow fullscreen-shell to use the new
weston_curtain infrastructure, rather than continuing to open-code the
common pattern of creating a surface and view consisting only of a solid
colour for the background of fullscreen surfaces which do not wholly
cover the output.

In doing this, the 'surface_subsurfaces_boundingbox()' function is
removed, as this has been duplicated between the fullscreen-shell and
the common helper 'library'.

There is no functional change within this commit, as the two functions
were identical, other than a change to the comment which identifies a
known bug within this helper.

Signed-off-by: Daniel Stone <daniels@collabora.com>